Electrochemical conversion of phenol to hydroquinone

ABSTRACT

The electrical efficiency of the electrochemical conversion of phenol to hydroquinone using lead anodes has been enhanced by preanodizing the lead anodes in an aqueous sulfuric acid solution containing various inorganic salts of chromium, manganese, iron, vanadium, or nickel.

United States Patent Inventor Frank 1-1. Covitz Lebanon, NJ.

Appl. No. 4,779

Filed Jan. 21, 1970 Patented Oct. 26, 1971 Assignee Union Carbide Corporation New York, N.Y.

ELECTROCHEMICAL CONVERSION'OF PHENOL T0 HYDROQUINONE 7 Claims, No Drawings U.S. Cl 204/78, 204/290 R, 204/57 Int. Cl ..C0lb 29/06, BOlk 3/04, C070 37/00 Field of Search 204/78, 280, 292, 290 R, 57

References Cited UNITED STATES PATENTS 2/1913 Reed 204/292 X 9/1938 Palfreeman 204/78 11/1938 Vagenius et a1. 204/78 7/1960 Gibson 204/57 4/1970 Covitz 204/78 Primary Examiner-F. C. Edmundson Atlorneys- Paul A. Rose, Aldo .I. Cozzi and Bernard F. Crowe ABSTRACT: The electrical efficiency of the electrochemical conversion of phenol to hydroquinone using lead anodes has been enhanced by preanodizing the lead anodes in an aqueous sulfuric acid solution containing various inorganic salts of chromium, manganese, iron, vanadium, or nickel.

ELECTROCHEMICAL CONVERSION OF PHENOL TO HYDROQUINONE BACKGROUND OF THE INVENTION This invention relates to the electrochemical conversion of phenol to hydroquinone and more particularly to improvements in the electrical efficiency of said reaction by pretreatment of the lead anodes.

Inasmuch as the cost of electrical power utilized in the electrochemical conversion of phenol to hydroquinone has a direct bearing on the economics of this process, it is highly desirable to hold side reactions which do not lead to the desired product to a minimum. If only one series of reactions were taking place in the instant process, that is, the electrochemical oxidation of phenol to p-benzoquinone at the anode and the reduction of the p-benzoquinone thus made to hydroquinone at the cathode, the ideal of 100 percent electrical efficiency would be achieved. However, as is the case with most electrochemical reactions this particular one is not that simple and so many competing reactions take place in the electrolysis cell which lowers the electrical efficiency for the preparation of hydroquinone from phenol significantly. Among the competing reactions which may take place in this reaction are the electrolysis of water to hydrogen and oxygen, the oxidation of phenol to carbon monoxide and hydrogen ion, the oxidation of phenol to carbon dioxide and hydrogen ion, the oxidation of phenol to oxalic acid, maleic acid and hydrogen ion and the like, as well as more complex reactions between p-benzoquinone, phenol, water or hydroquinone leading to polynuclear phenols, trihydroxy benzenes and hydroxy phenoxy hydroquinones as well as to tar formation.

SUMMARY OF THE INVENTION It has now been found that the electrical efficiency of the electrolytic preparation of hydroquinone from phenol can be improved by preanodizing the lead anode in an aqueous sulfuric acid solution containing inorganic salts such as halides of metals of group VIB or the nickel family ofgroup VIII, sulfates of metals of group VIB, group VlIB, chromium, and the iron family of group VIII of the Deming Periodic Table or ammonium or alkali metal vanadates.

DESCRIPTION OF THE INVENTION The term electrical efficiency as used in the practice of this invention is equal to the fraction 4(H Q+Q)/F,, where:

H O is the rate of formation of hydroquinone in moles per hour, Q is the rate of formation of quinone in moles per hour and F, is the coulombic input in Faradays per hour. This relationship is derived from the stoichiometry of the electrode reaction taking place at the anode in the electrochemical conversion of phenol to hydroquinone which is depicted below.

STOlCI-IIOMETRY OF ANODIC REACTION OH I ll The term H 0 is included in the above expression for electrical efficiency even though only the anodes are modified in this invention because hydroquinone can only arise through a quinone intermediate by reduction at the cathode since no way is known of proceeding directly from phenol to hydroquinone at the anode.

In order to compare the efficiencies of various metal salts for the improvement of anodic electrical efficiencies in the electrochemical conversion of phenol to hydroquinone through the use of lead electrodes, the operating conditions were held constant and only qualitative changes were made proceeding the nature of the metal salt used for the pretreatment. In one series of experiments the pretreatment consisted in immersing the electrode in an electrolyte solution of 0.1 weight percent of the test salt dissolved in 3 percent aqueous sulfuric acid and anodizing for 10 minutes at a current density of 0.7 amperes per square decimeter at 25 C. The conditions for the electrolysis of phenol to hydroquinone consisted of using an undivided cell in a batch operation with a phenol concentration of 3 percent per cc. of 3 percent aqueous sulfuric acid and electrolyzing the resultant mixture at a current density of 4 amperes per square decimeter at 40 C. until 10 percent by weight of the phenol charge had been electrolyzed. Then by substituting the values for coulombic input and rate of formation of hydroquinone and quinone in moles per hour in the equation supra the anodic electrical efficiencies for each test metal salt were determined.

It was found that under these conditions the average for a series of about eight runs using lead electrodes which were not pretreated or modified, the electrical efiiciency was about 57 percent. The results obtained with a number of different metal salts are presented in the table below.

TABLE I Effect of Anodic Pretreatment on Electrical Efficiency of the Electrolysis of Phenol to Hydroquinone Salt Electrical Efficiency, 2

4 CrCl MnSO, NiCl 4 1): NaVO, Control (No Salt) NiSO. C050, Cd( NO, )2 )1 AgNO,

64 NH, ,vo, so mm,

58 CdSO,

periment described above.

Although not wishing to be bound by any particular theory or explanation, it is postulated that the general effect of preanodization in the presence of metal salts is probably due to adsorption and/or oxidation of the metal, thus altering the surface characteristics of the lead dioxide deposit on the anodes.

A further extension of this invention was uncovered by the discovery that lead anode attrition or corrosion can be lessened by preanodizing said lead anode in an aqueous sulfuric acid solution before carrying out the electrolysis of phenol. The corrosion was measured by observing the loss in weight of a lead anode with and without preanodizing in a 40 percent (weight/volume) aqueous sulfuric acid solution. Controls where no preanodization was used resulted in loss of from 8 to 15 g./l00 hr./dm. oflead anode during the phenol electrolysis reaction. In contrast where the lead anode had been preanodized in 40 percent aqueous sulfuric acid at 25 C. for l0 minutes the lead anode weight loss was cut to 4 g./l00 hr./dm

A preferred embodiment of this invention is one where the preanodization is carried out using the preferred salts described supra in aqueous sulfuric acid solutions containing about 3 to 40 percent by weight of sulfuric acid and even more preferred using aqueous sulfuric acid solutions containingabout 30 to 40 percent by weight of sulfuric acid.

Although the invention has been described with some degree of particularity, it is understood that many changes and l modifications can be made without departing from the spirit and scope of the invention. 7 What is clauned is:

1. In the method of preparing hydroquinone which comprises the steps of:

a. electrolyzing an aqueous solution containing from about 0.5 to 4 percent by weight of a phenol and about 1 to 35 percent by weight of an electrolyte at a temperature of about 25 to 100 C., a pH of less than about 4, an anode DC potential of at least about +0.9 volts in reference to a saturated calomel electrode, a cathode DC potential more negative than +0.4 volts in reference to a saturated calomel electrode, and a current density of at least 4 amperes per square decimeter until up to about 80 percent by weight of the phenol has been electrolyzed to hydroquinone wherein both anode and cathode are fabricated from lead; and

b. recovering the hydroquinone from the aqueous solution,

the improvement which comprises preanodizing said lead anode in a aqueous sulfuric acid solution of about 3 to 40 ercent b y weight sulfuric acid containing inorganic salts selected from the group consisting of chromic sulfate,

chromic chloride, manganous sulfate, ammonium varradate, nickelous chloride, nickelic chloride and ferric sulfate at a concentration of about 0.1 percent. 2. Method claimed in claim 1 wherein the salt is chromic sulfate.

3. Method claimed in claim 1 wherein the slat is chromic chloride.

4. Method claimed in claim l wherein the salt is manganous sulfate.

5. Method claimed in claim I wherein the salt is ammonium vanadate.

6. Method claimed in claim 1 wherein the salt is nickel chloride 7. Method claimed in claim 1 wherein the aqueous sulfuric acid solution contains about 30 to 40 percent by weight sulfuric acid. 

2. Method claimed in claim 1 wherein the salt is chromic sulfate.
 3. Method claimed in claim 1 wherein the salt is chromic chloride.
 4. Method claimed in claim 1 wherein the salt is manganous sulfate.
 5. Method claimed in claim 1 wherein the salt is ammonium vanadate.
 6. Method claimed in claim 1 wherein the salt is nickel chloride.
 7. Method claimed in claim 1 wherein the aqueous sulfuric acid solution contains about 30 to 40 percent by weight sulfuric acid. 